In the production of foamed resins in the prior art, a method is known of producing a foamed resin by an extruder using a polymer (for example, in the case of polystyrenes, the concentration of volatile substances, such as the remaining unreacted monomer and solvents, is approximately on the order of 3,000 to 400 ppm), and kneading the polymer with, as a blowing agent, a hydrocarbon such as propane, butane, and pentane, or a halogenated hydrocarbon such as trichlorofluoromethane.
On the other hand, in polymer resins and polymer foamed products, the demand for lowering the level of the concentration of remaining volatile substances has become severe, and it is desired that their concentration be reduced to about 100 ppm or less.
However, in the above prior method for the production of foamed resins, volatile substances, such as the unreacted monomer and solvents remaining in the polymer that is used in the production of a foam, can hardly be reduced to a level lower than a limit of approximately 400 ppm during conventional steps of producing the polymer. Thus, the molded or foamed product obtained from such a polymer has difficulty in meeting the market demand that the concentration of the residual volatile substances be not more than the above mentioned value. Further, in the prior art methods since the steps in the production of a foamed resin are carried out independent of the removal of volatile substances from the polymer (i.e., purification of the polymer), the steps involved in making the foam into a final product become complicated, and the prior art method has a problem in view of economy, such as the required energy.